Re: [PATCH v3 5/6] ext4: introduce direct IO write path using iomap infrastructure

Jan Kara
2019-09-24T14:13:10Z

On Tue 24-09-19 20:29:26, Matthew Bobrowski wrote:
> On Mon, Sep 23, 2019 at 11:10:11PM +0200, Jan Kara wrote:
> > On Thu 12-09-19 21:04:46, Matthew Bobrowski wrote:
> > > +static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
> > > +{
> > > + ssize_t ret;
> > > + size_t count;
> > > + loff_t offset = iocb->ki_pos;
> > > + struct inode *inode = file_inode(iocb->ki_filp);
> > > + bool extend = false, overwrite = false, unaligned_aio = false;
> > > +
> > > + if (!inode_trylock(inode)) {
> > > + if (iocb->ki_flags & IOCB_NOWAIT)
> > > + return -EAGAIN;
> > > + inode_lock(inode);
> > > + }
> > > +
> > > + if (!ext4_dio_checks(inode)) {
> > > + inode_unlock(inode);
> > > + /*
> > > + * Fallback to buffered IO if the operation on the
> > > + * inode is not supported by direct IO.
> > > + */
> > > + return ext4_buffered_write_iter(iocb, from);
> > > + }
> > > +
> > > + ret = ext4_write_checks(iocb, from);
> > > + if (ret <= 0) {
> > > + inode_unlock(inode);
> > > + return ret;
> > > + }
> > > +
> > > + /*
> > > + * Unaligned direct AIO must be serialized among each other as
> > > + * the zeroing of partial blocks of two competing unaligned
> > > + * AIOs can result in data corruption.
> > > + */
> > > + if (ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS) &&
> > > + !is_sync_kiocb(iocb) && ext4_unaligned_aio(inode, from, offset)) {
> > > + unaligned_aio = true;
> > > + inode_dio_wait(inode);
> > > + }
> > > +
> > > + /*
> > > + * Determine whether the IO operation will overwrite allocated
> > > + * and initialized blocks. If so, check to see whether it is
> > > + * possible to take the dioread_nolock path.
> > > + */
> > > + count = iov_iter_count(from);
> > > + if (!unaligned_aio && ext4_overwrite_io(inode, offset, count) &&
> > > + ext4_should_dioread_nolock(inode)) {
> > > + overwrite = true;
> > > + downgrade_write(&inode->i_rwsem);
> > > + }
> > > +
> > > + if (offset + count > i_size_read(inode) ||
> > > + offset + count > EXT4_I(inode)->i_disksize) {
> > > + ext4_update_i_disksize(inode, inode->i_size);
> > > + extend = true;
> > > + }
> >
> > This call to ext4_update_i_disksize() is definitely wrong. If nothing else,
> > you need to also have transaction started and call ext4_mark_inode_dirty()
> > to actually journal the change of i_disksize (ext4_update_i_disksize()
> > updates only the in-memory copy of the entry). Also the direct IO code
> > needs to add the inode to the orphan list so that in case of crash, blocks
> > allocated beyond EOF get truncated on next mount. That is the whole point
> > of this excercise with i_disksize after all.
> >
> > But I'm wondering if i_disksize update is needed. Truncate cannot be in
> > progress (we hold i_rwsem) and dirty pages will be flushed by
> > iomap_dio_rw() before we start to allocate any blocks. So it should be
> > enough to have here:
>
> Well, I initially thought the same, however doing some research shows that we
> have the following edge case:
> - 45d8ec4d9fd54
> and
> - 73fdad00b208b
>
> In fact you can reproduce the exact same i_size corruption issue by running
> the generic/475 xfstests mutitple times, as articulated within
> 45d8ec4d9fd54. So with that, I'm kind of confused and thinking that there may
> be a problem that resides elsewhere that may need addressing?

Right, I forgot about the special case explained in 45d8ec4d9fd54 where
there's unwritted delalloc write beyond range where DIO write happens.

> > if (offset + count > i_size_read(inode)) {
> > /*
> > * Add inode to orphan list so that blocks allocated beyond
> > * EOF get properly truncated in case of crash.
> > */
> > start transaction handle
> > add inode to orphan list
> > stop transaction handle
> > }
> >
> > And just leave i_disksize at whatever it currently is.
>
> I originally had the code which added the inode to the orphan list here, but
> then I thought to myself that it'd make more sense to actually do this step
> closer to the point where we've managed to successfully allocate the required
> blocks for the write. This prevents the need to spray orphan list clean up
> code all over the place just to cover the case that a write which had intended
> to extend the inode beyond i_size had failed prematurely (i.e. before block
> allocation). So, hence the reason why I thought having it in
> ext4_iomap_begin() would make more sense, because at that point in the write
> path, there is enough/or more assurance to make the call around whether we
> will in fact be able to perform the write which will be extending beyond
> i_size, or not and consequently whether the inode should be placed onto the
> orphan list?
>
> Ideally I'd like to turn this statement into:
>
> if (offset + count > i_size_read(inode))
> extend = true;
>
> Maybe I'm missing something here and there's actually a really good reason for
> doing this nice and early? What are your thoughts about what I've mentioned
> above?

Well, the slight trouble with adding inode to orphan list in
ext4_iomap_begin() is that then it is somewhat difficult to tell whether
you need to remove it when IO is done because there's no way how to
propagate that information from ext4_iomap_begin() and checking against
i_disksize is unreliable because it can change (due to writeback of
delalloc pages) while direct IO is running. But I think we can overcome
that by splitting our end_io functions to two - ext4_dio_write_end_io() and
ext4_dio_extend_write_end_io(). So:

WARN_ON_ONCE(i_size_read(inode) < EXT4_I(inode)->i_disksize);
/*
* Need to check against i_disksize as there may be dellalloc writes
* pending.
*/
if (offset + count > EXT4_I(inode)->i_disksize)
extend = true;

...
iomap_dio_rw(...,
extend ? ext4_dio_extend_write_end_io : ext4_dio_write_end_io);

and ext4_dio_write_end_io() will just take care of conversion of unwritten
extents on successful IO completion, while ext4_dio_extend_write_end_io()
will take care of all the complex stuff with orphan handling, extension
of inode size, and truncation of blocks beyond EOF - and it can do that
because it is guaranteed to run under the protection of i_rwsem held in
ext4_dio_write_iter().

Alternatively, we could also just pass NULL instead of
ext4_dio_extend_write_end_io() and just do all the work explicitely in
ext4_dio_write_iter() in the 'extend' case. That might be actually the most
transparent option...

But at this point there are so many suggestions in flight that I need to
see current state of the code again to be able to tell anything useful :).

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR